A side light type area light source is often applied to a liquid crystal display (LCD) device because such a side light type area light source is suitable for an LCD device with a technical feature of particularly thin thickness. The side light type area light source includes a light guide, a cold-cathode fluorescent tube and an optical sheet. The cold-cathode fluorescent tube is set on a side of the light guide, light emitting from the cold-cathode tube is applied to a light incident surface provided on the side of the light guide, and the light applied is converted into area light through the optical sheet such as a reflective sheet.
Since light emitting diode (LED) devices has been recently improved significantly in efficiency, so that the LED devices are now comparable to cold-cathode tubes, such LED devices are used for light sources of mobile phones. As disclosed in Japanese Unexamined Patent Publication No. 2003-132722, a saw-tooth prism group is provided at a side portion of a light guide and LED devices are arranged at a side portion of the prism group so that resultant light from point light sources of the LED devices can become area light efficiently.
The saw-tooth prism of the area light source set forth above can convert the light from the LED devices into the area light efficiently. The area light source, however, has a problem that the LED devices are locally high in brightness in their vicinities. As a result, uneven brightness is observed on an LCD device provided with the area light source.
The present invention provides an area light source with the suppression of uneven brightness in the case that point light sources of LED devices are arranged along a side of a light guide.